Sorption of plutonium

The sorption of plutonium on a variety of the common minerals of igneous rocks under groundwater conditions is illustrated in Figure 3 (57, 58, 59). For comparison the sorption of other acti-... 

Sorption of plutonium (l.fixlO-11 M) and americium (2xl0-9 M) in artificial groundwater (salt concentration 300 mg/liter total carbonate 120 mg/liter Ref. 59) on some geologic minerals, quartz, biotite, o apatite, o attapulgite, montmorillonite. Dashed lines indicate the range for major minerals in igneous rocks. Experimental conditions room temperature, particle size 0.04-0.06 mm, solid/liquid ratio 6-10 g/1, aerated system, contact time 6 days. 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

Lu, N., Cotter, C. R., Kitten, H. D., Bentley, J. TRIAY, I. R. 1998. Reversibility of sorption of plutonium-239 onto hematite and goethite colloids. Radiochimica Acta, 83, 167-173. 

In previous work (l.> > ) it was found that the kinetics of sorption was an important parameter affecting the migration of nuclides in geologic media. For example, in experiments designed to measure the kinetics of reaction for radionuclides in solution with tablets of rock, it was found that periods from several minutes to several hours were required for the radionuclides to reach steady state concentrations on the rock tablets and in the solutions. Figure 1 shows the reaction curves found for the sorption of plutonium and americium from solution by a tablet of granite. The reaction rates for the sorption of plutonium and americium from solution are not the same, and both require a number of hours to reach steady state concentrations. 

Studies of the sorption of plutonium are complicated by the high redox reactivity of plutonium. Sorption of Pu(V) by pure aluminosilicates and oxyhydroxide phases is usually characterized by initial rapid uptake followed by slow irreversible sorption and may represent a reductive uptake mechanism catalyzed by the electrical double layer of the mineral surface (Turner et al., 1998 Runde et al., 2002a). In Yucca Mountain waters, the ranges for Pu(V) uptake by hematite, montmorillonite, and silica colloids were 4.9xl0 mLg to 1.8 X 10 mL g 5.8 X 10 mL and 8.1 X 10 mL g, respectively. These are much higher than those observed for Np(V) in the same waters as described previously. High surface redox reactivity for... 

Sorption of plutonium traces onto alumina from uranyl nitra-te solutions has been investigated. Several methods have been previously proposed for the recovery of plutonium traces from reprocessing solutions. Those methods include ion exchange (1 2), solvent extraction and, extraction chromatography 0,6). 

Table II. Sorption of Plutonium onto Silica in High and Low Ionic Strength Solutions at pH 5 and 7.1...

Table III. Sorption of Plutonium by Different Size Silica Particles ...

In considering the sorption of plutonium onto a solid surface, such as silica, in an aqueous solution, the mechanism of interaction is of par-... 

The partitioning of plutonium from surface water to sediments in freshwater and marine environments depends on the equilibrium between plutonium(IV) and plutonium(V), and the interaction between plutonium(IV) in solution and plutonium sorbed onto sediment particle surfaces (NCRP 1984). Sorption onto marine clays was found to be largely irreversible (Higgo and Rees 1986). Higgo and Rees (1986) also found that the initial sorption of plutonium onto clays was effective in removing most of the plutonium species that would be able to sorb onto the clay. When sorption to carbonate marine sediments was investigated, it was found that some desorption from the surface would also occur. This behavior was due to the presence of plutonium carbonate complexes on the sediment surfaces which were sorbed less strongly than plutonium dioxide... 

For the first purification of plutonium in the processing of irradiated nuclear fuels, an anion-exchange process has been widely used [202]. In this process, complex formation of plutonium(iv) with nitrate is utilized in order to remove the last traces of uranium (present as uranyl(vi)) and fission products (primarily lanthanides). In this system, the maximum sorption of plutonium (iv) occurs at a nitric acid concentration of 7.2 m. The process is run at 60°C. At lower temperatures, the sorption is too slow at higher temperatures, the distribution ratio becomes more unfavorable and the resin is more liable to deteriorate. Under the conditions chosen, neither uranyl(vi) nor lanthanides are sorbed. The elution of plutonium(iv) is readily achieved by dilute (0.7 m) nitric acid. The weak point of the process is the limited resistance of organic ion exchangers to chemical attack and to high doses of radiation, already discussed in Section 21.6.1. These difficulties can be overcome, at least partly, by careful selection of the resin to be used. 

These studies have included sorption of plutonium, uranium and thorium onto a wide range of BVG rock types, both matrix tuff and fracture mineral assemblages. Some typical results are shown in Figs 7 to 9. The effect of organic material on radionuclide sorption varied... 

Data for the ACDP solution and the highest ISA concentration are generally very similar, confirming that ISA is an important component of the ACDP solution. However, this is not the case for the sorption of plutonium and thorium at neutral pH, where ACDP solution appears to have a much greater... 

C. J. Williams, S. J. 1995. Sorption of plutonium and americium on repository, backfill and geological materials relevant to the JNFL low-level radioactive waste repository at Rokkasho-Mura. Scientific Basis for Nuclear Waste Management XVIII, Materials Research Society Symposium Proceedings, 353, 957-964. 

---